The requirement that Newton's laws be the limit of general relativity for small v forces lambda to be very small. Setting it equal to zero gives all the correct predictions for the motions of planets to within measurable accuracy. Put another way, if 0, then experimental data shows that it must be very small indeed. Also, we could take that term over to the right-hand side of the equation and incorporate it into the stress-energy tensor, thus regarding -gab/8¹ as the stress-energy tensor of empty space.
Following is an excerpt from an article in Scientific American (September, 1996, p. 22):
... Yet the cosmological constant itself is a source of much puzzlement. Indeed, Christopher T. Hill of Fermilab calls it "the biggest problem in all of physics." Current big bang models require that lambda is small or zero, and various observations support that assumption. Hill points out, however, that current particle physics theory predicts a cosmological constant much, much greater-by a factor of at least 1052, large enough to have crunched the universe back down to nothing immediately after the big bang. "Something is happening to suppress this vacuum density," says Alan Guth of MIT, one of the developers of the inflationary theory. Nobody knows, however, what that something is ...
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